Almost as soon as the American painter, Samuel F. B. Morse, developed, with the invention of the telegraph, the first electrical communications system in 1832, contemporaries and those that followed in their footsteps were quick to see the potential of the new medium and began to dream and work toward the implementation of video transmission over an electrical channel.
Perhaps one of the first practical implementations of picture transmission is reflected by “fac simile” systems patented in the 1860s. In these systems, a candle, illuminating an image which was to be transmitted, provides an image which was focused onto rotating disk with strategically placed holes, similar to that which would later be used by Nipkow. The optical output of the disk was passed to a photoelectrical device which sent an electrical signal over a transmission line to a receiver. The receiver comprised a rotating metal drum with a sheet of paper wrapped around it. The paper was impregnated with gunpowder and as the drum rotated, the end of a fine metal wire scanned across the length of the drum, defining a helix along the surface of the drum. The signal from the transmission line drove an electrical spark between the end of the wire and the drum when certain video levels were reached, resulting in the selective ignition of the gunpowder and associated localized burning of the image into the paper. Such gunpowder-based receivers continued to be used in commercial applications into the 1950's for the purpose of transmitting so-called “wirephotos” in the newspaper business. Indeed, the phrase “burning a copy” is still used today when referring to the recording of information in any medium.
By 1884, Paul Nipkow proposed the transmission of a live moving picture by dividing a visual scene into a plurality of frames. The frames were, in turn, divided into a matrix of raster lines implemented by scanning an image with a disk including a plurality of strategically placed holes. The holes were positioned to define the raster lines at the position of the image when the disk was rotated. The light passed by the disk thus formed a first video signal comprising a series of light intensity reading corresponding to a sequence of points in a well-defined order that could be reassembled at the other end of a transmission channel.
While binary transmissions, such as those used by Morse, impose minimal requirements on the transmission line, increasing bandwidth in the frequency domain will increase the criticality of the transmission line characteristic. Nevertheless, digital transmission is substantially immune to noise and transmission line characteristics as long as noise and distortion remain below certain threshold limits, after which errors avalanche substantially destroying communication. An example of this is found in wireless telephone communication, where quality tends to remain at a very acceptable level until the threshold of the system is exceeded and the signal simply disappears.
Conversely, analog transmission introduces proportional sensitivity to transmission line noise and characteristics from the lowest levels of noise and distortion, but without the cataclysmic breakdown of the medium at higher levels of noise. Compare the above example of a connection in a cellular telephone system to the “snowy” picture received from a distant television station, which, despite its inclusion of numerous elements of noise, continues to provide a reasonable facsimile of the transmitted image.
High-density video information remains the most information dense analog signal in common use. The most demanding video applications involve high-resolution computer video. Not surprisingly, these high-resolution computer video signals, unlike audio signals and far more critically than conventional NTSC television signals, do not travel well over common transmission lines, such as unshielded twisted-pair lines. Complications include attenuation of the signal along the length of the line and, due to the interaction of irregular capacitance and inductive components distributed along the length of the line, a frequency domain characteristic which can cause severe degradation in two aspects of the video signal.
The first aspect of the problem is the distortion of the synchronization pulses which insure alignment of the raster lines with respect to each other and registration of successive fields and frames in the video signal. This problem can become particularly severe in the case of relatively long transmission lines. The second aspect of the problem is the distortion or degradation of the video signal itself. Such signals generally comprise both multiple color channels. The speed with which video information varies, combined with phase and amplitude varying effects of distributed conductive, capacitance and resistive components of the transmission line, can cause substantial degradation of a video signal. This degradation is frequency and distance dependent. More particularly, the losses increase geometrically with frequency and distance. Rise time and time domain performance are progressively impacted as the transmission line length increases. As the rise times begin to exceed some small multiple of single pixel duration time, the image begins to suffer from aperture distortion with a horizontally smeared appearance. Image detail becomes reduced and quickly degrades to the point where small text becomes illegible. Areas of the image where there are transitions from dark to light and light to dark, become ill-defined and can quickly cause viewer fatigue.
Quantitatively, losses over unshielded twisted pair are approximated by the expression:L=0.988(1.967vf)+0.023f+(0.05/vf),where L represents losses in dB per 100 meters, f is frequency and v is a constant.
Traditionally, transmission lines for carrying video almost always took the form of coaxial cable. Coaxial cable, invented in 1929 by Lloyd Espenschied and Herman Affel of Bell Labs, generally comprise a twisted or solid copper wire axial conductor surrounded by a flexible plastic dielectric insulator having a generally circular cross-section. The insulator is, in turn, surrounded by a tubular woven copper wire mesh, which forms a cylindrical shield around the axial copper member. The structure of the cable is completed by an outer flexible insulator skin comprising a layer of rubber or similar synthetic material. Such a coaxial transmission line provides a balanced transmission channel having low losses, minimal distortion, and a well-defined characteristic substantially limited to signal attenuation, provided that either the output impedance of the driving source is matched to the impedance of the coaxial cable or the length of the cable is calculated to compensate for any mismatch.
Commercially available coaxial cables do an excellent job of transmitting high-resolution computer video signals over moderate distances. However, as with any other metallic transmission line, provision of accurate signal transmission over more than a few hundred feet, requires active equalization to compensate for the erosion of bandwidth. Coaxial cables also suffer from a number of specific drawbacks. First of all, coaxial cable is physically stiff, heavy and hard to handle. Termination of the coaxial cable with an appropriate connector is also a relatively, complicated and time-consuming task which is not well-suited to mechanization, involving selective removal of different lengths of axial central conductor, dielectric insulator, woven shield and outer insulative skin. Termination of coaxial cable in the field is prohibitively expensive, and the quality of the terminal installation is difficult to control. For computer type video, which requires a total of five signals, five individual cables must be used to carry the information. If five such cables are to be used, the length of each one must be tightly controlled to reduce the effects of varying signal propagation times. Not doing so will cause a deterioration of color convergence of the displayed image. Since coaxial cable is very expensive compared to other cable products, it becomes quite unattractive as a universal baseband video transportation and distribution solution.
The information age has fostered the pursuit of bandwidth and granular connectivity. With an eye toward cost it was nearly inevitable that the communications industry would distill the requirement down to a simple, structured approach for the physical interconnect layer. Hence the development and adoption of structured cabling or unshielded twisted pair (“UTP”).
Accordingly, strong prejudices in the computer industry in favor of twisted-pair and other inexpensive and easy to handle and terminate transmission lines have resulted in the supplantation of coaxial cable by twisted-pair.
In relatively short lengths, for example one to fifteen meters twisted pair performance in transmitting video signals ranges from excellent to fair, respectively. Longer length traditionally require the use of coaxial cable if high quality signal transmission characteristics are desired.
Some attempts have been made to address the more serious aspects of long length twisted pair transmission lines. More particularly, it has been recognized that synchronization errors are equally as serious as distortions in image detail resolution.
More particularly, degradation tends to exhibit itself as a loss of resolution and can be tolerated when the loss of details of the picture are not so critically evaluated by the eye and, sometimes, or not critical to a general understanding of the transmitted image. For example, degraded pictures of people in motion, while they may not have a high-quality appearance, will not be disturbing to the viewer. On the other hand, the degraded image of small font type, or, for example, hair of a model on whom a hair treatment product is being demonstrated, may make comprehension of the content of a video transmission difficult or impossible.
On the other hand, distortions in synchronization signals result in loss of registration between frames and/or loss of alignment between lines in a rasterized image. Worse still, with today's digital display technology, a momentary loss of sync information causes the complete loss of an image for many seconds while the equipment falls into “mute condition” and struggles to re-map the incoming signal. This sort of loss in signal quality results in a highly visible and disturbing degradation in displayed image quality. Such poor performance is not tolerated by the eye and is absolutely unacceptable by any commercial standards.
In an effort to address the more serious problems involved in accurately sending synchronization information over an unshielded twisted-pair cable and also to find a practical way to accommodate the transport of five signals on four pairs of wire, various approaches have been proposed. One of the most common techniques is to transport the baseband signals over two discrete “phantom circuits.” John Joseph Carty, a Bell engineer, invented the concept of phantom or side circuits in 1886. By driving the center tap of balanced loading coils in each of the two circuits, the technique enable the carriage of three separate telephone signals over two circuits. This was an excellent solution for voice signals and can even work fairly well for video sync signals over UTP cable for short distances.
For video sync transmission, the technique suffers from poor noise immunity and relies upon an uncontrolled impedance mode of the cable, namely the impendence between alternate pairs. To gain reasonable noise immunity, the threshold levels must be fairly high in amplitude and as such, cause the transition points to become sensitive to cable length. This is due to the relationship between the cable high frequency attenuation and length. The longer the cable, the slower the rise time and the later the sync pulse transition occurs. Therefore, as the cable length increases, the sync transition is delayed, and the presented image shifts very noticeably to the right side of the viewing area. Another technique relies on encoding the two sync signals, combining them and then transporting them together on the fourth pair. This technique provides good noise immunity but, unless a separate equalizer is used to overcome the losses encountered in the cable, similar time displacement errors occur as with the phantom circuit technique. Also, with this technique, the difference in propagation time between the “fourth pair” and the other pairs exacerbates the time displacement issue. Lastly, using one entire pair to transport sync signals yields poor economy since the pair cannot easily be used for any other more valuable purpose.
Clearly, a more desirable approach is to transport the sync signals along with the video signals where the transit time through the cable for the sync and video signals is identical. This approach also allows for an economically appealing architecture, where the sync signals are processed by the same cable loss equalizer as the video. Doing so restores the rise time of the sync pulse, thus eliminating the time displacement concern.
A significant challenge in making this approach successful is to remove the sync signals from the video prior to their interface with the display. Failure to do so will result in unpredictable image presentation quality. Part of the inventive solution is to sum the sync signals along with the video signal for transportation through the cable and then remove said sync signal with a high performance clamping circuit prior to outputting it to the display before transmission through the cable with relatively high magnitude synchronization signals.
While such high magnitude synchronization signals/unshielded twisted-pair systems provide a good alternative, in many applications, to coaxial transmission lines, the degraded nature of the images makes them undesirable for many applications.